A radio communication device in a radio communication system has been desired to be highly efficient from a view point of size reduction and power saving of the device.
A power amplifier included in the radio communication device is used in a saturation region where the efficiency of the device is generally high. However, a nonlinear distortion is increased if the power amplifier is used near the saturation region. Distortion compensating processing has been known as a technique for suppressing the nonlinear distortion and reducing an Adjacent Channel Power (ACP). A Feed Forward Amplifier (FF amplifier) that compensates a distortion of a main amplifier by a sub-amplifier, a Cartesian method for compensating the distortion by feeding back an amplifier output, and a Pre-distortion method for compensating the distortion by inputting an inverse distortion characteristic signal of the amplifier into the amplifier are examples of the distortion compensating processing. Regarding the Pre-distortion method, a digital pre-distortion method with a feedback loop that uses a series or a Look Up Table (hereinafter referred to as LUT) has been known. According to the pre-distortion method using the LUT, the distortion compensation coefficient used for the distortion compensating processing is stored in the LUT, and the distortion compensation coefficient is updated based on a transmission signal and a feedback signal.
On the other hand, in the power amplifier, a phenomenon called a memory effect is generated. The memory effect is a phenomenon in which an output signal corresponding to an input signal at a time is influenced by a past input signal. Therefore, the distortion compensation device is desired to compensate a nonlinear distortion in the above-described power amplifier and to compensate the influence of the memory effect in the power amplifier.
Patent Document 1 discloses a pre-distorter that includes a plurality of LUTs corresponding to a plurality of bands and uses a forgetting coefficient to update the distortion compensation coefficient. Patent Document 2 discloses an amplifier that compensates a distortion by using a forgetting coefficient to update the distortion compensation coefficient. Patent Document 3 discloses a pre-distortion device that includes a plurality of LUTs and performs distortion compensation by synthesizing the distortion compensation coefficient output from the plurality of LUTs.